Contrast reduction using energy dependent intensifying screens

The speed of intensifying screens, in particular rare earth screens, increases with beam energy. Using such a screen, the measured contrast of an aluminium disc, with optical densities in the linear region of the characteristic curve, was found to be significantly and consistently less than that calculated from the gamma of the film-screen combination for all tube potentials from 40 to 140 kVp. This contrast reduction effect can be explained in terms of the filtering action of the disc which increases the effective energy of the beam. Hence, the film-screen combination is exposed to X-rays which have different effective energies. This results in two characteristic curves of different speed. The contrast of the disc image is not dependent on the gamma of either curve, but an effective gamma which links the two curves. This effective gamma is less than the gamma of either curve. Contrast calculated using the effective gamma gave good agreement with the measured contrast of the disc image over the tube potential range examined.